1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus. More particularly, the present invention relates to a process cartridge capable of readily rendering a developing nip that is otherwise separated during shipping, distribution, and storage while circulated in the market, and an image forming apparatus having the same.
2. Description of the Related Art
In a conventional image forming apparatus which adopts an electrophotographic process, such as a laser printer, a copying machine or a facsimile, a process cartridge is used to combine a photoconductive body and a processing means into one cartridge capable of being installed into and removed from a main body of the image forming apparatus. This process cartridge is intended to simplify replacement of parts by combining various rollers of approximately the same life span including, for example, the photoconductive body and the developing roller, and a certain amount of toner into a single cartridge. An internal configuration of important parts of such a conventional process cartridge is illustrated in FIGS. 1 and 2.
In the cartridge shown in FIG. 1, a laser beam is scanned on a surface of a photoconductive drum 10 rotatably installed between two side plates 14, 16, to form a latent electrostatic image thereon. Then, a developing roller 12 in contact with the photoconductive drum 10 supplies the toner thereon to change the latent electrostatic image into a visible image.
The conventional process cartridge adopts a “contact development method”, which usually has advantages of low noise, low power consumption, compactness in size and high definition, compared with a non-contact development method. However, in the contact development method, the amount of a toner supplied to the photoconductive body for developing the latent electrostatic image is very sensitive to the size of the developing nip D (see FIG. 2), i.e., an amount of contact surface between the photoconductive body and the developing roller. Therefore, to produce a high quality printed image using the contact development method, a predetermined developing nip D should be maintained between the developing roller and the photoconductive body. According to experimental results, a practically allowable range of the developing nip D is about +0.1 mm to about +0.2 mm in terms of an overlapping amount in the radial distance between the developing roller and the photoconductive body (i.e., an overlapping amount of outer radii of the developing roller and the photoconductive body on a line connecting the central axes of the developing roller and the photoconductive body). If the developing nip is outside of this range, there will be many significant deficiencies in the printed image. In practice, the conventional process cartridge has a specific developing nip fixed at the time of manufacture before it is initially put into circulation in the market; however the cartridge may not actually be used by a user until after a long period of time. Therefore, since the process cartridge may be in circulation for some time while the developing roller and the photoconductive body are in pressure contact with each other due to the initially set developing nip, several problems may arise, for instance the developing roller may be deformed by the pressure from the photoconductive body or the toner in the developing nip D may adhere to either the photoconductive body, the developing roller or both. Further, horizontal white lines or horizontal black lines may appear in the printed image due to migration between the photoconductive body and the developing roller.
To solve such problems, technologies for reducing the pressure on the developing roller due to the presence of the developing nip or reducing the amount of overlapping area have been developed. But, problems with these techniques, such as white voids appearing in both side portions of an image, remain unresolved. Therefore, to completely separate the developing nip when the cartridge is not in operation is one of the best ways of fundamentally solving the image defect problems caused by the developing nip. The developing nip is the interference between the photoconductive body and the developing roller, however when the bodies are separated so that there is no overlap, the developing nip itself is said to be separated.
A representative example of an image forming cartridge capable of separating the developing nip is disclosed in Japanese Patent Publication No. 2003-323017 (U.S. Pat. No. 6,882,811). FIGS. 3 and 4 illustrate such a process cartridge.
The cartridge shown in FIGS. 3 and 4 includes a plate 14; a photoconductive body 10 supported to be rotatable with respect to the plate 14; an eccentric cam 20 installed in the plate 14 and positioned in a first position A (operation position) or a second position B (retreat position); a developing roller 12 disposed to be rotatable with respect to the eccentric cam 20; and a lever connector 22 for positioning the eccentric cam 20 in the first or the second position. In the aforementioned cartridge, the eccentric cam 20 is moved to the first position A to cause the developing roller 12 to form a predetermined nip (indicated by dotted lines in FIG. 4) or to the second position B to cause the developing roller 12 to separate from the photoconductive body 10 (indicated by solid lines in FIG. 4). Therefore, when the cartridge is in an inoperative state, the developing nip between the photoconductive body 10 and the developing roller 12 can be separated by positioning the developing roller 12 in the retreat position.
However, a drawback in the aforementioned technology, is that since the eccentric cam 20 moves the center axis of the developing roller 12 from a point sh1 to a point sh2 as shown in FIG. 4, thus moving the developing roller 12 directly, a supply roller 15 and a blade 17 adjacent to the developing roller 12 are also affected. In addition, this may have an adverse effect on various gears coupled to the developing roller, the supply roller and the like, i.e., a gear train needed to drive the rotation of these cartridge components. Further, a seal sponge 18 or a pressing member for pressing a blade 17 is additionally required to prevent toner leakage due to the movement of the developing roller 12.